The present invention relates to a metal gasket to be installed between two engine members, such as a cylinder head and a cylinder block of an engine, to seal the area there between.
The cylinder head gasket is tightened by head bolts once the cylinder head gasket is installed between the cylinder head and the cylinder block (cylinder body) of an automobile engine (internal combustion engine). The cylinder head gasket serves to seal fluids such as combustion gas, oil, coolant water, and so on.
Recently, as a means of reducing the weight of engines, engine materials have been aluminum-alloyed or otherwise reduced in weight so that low rigidity of the engine is rapidly resulting. As a result, in terms of the sealing condition of a modern head gasket for a multi-cylindered engine, there is a tendency toward increasing failures due to local combustion gas leak. A main reason for the aforementioned failures is either the local low rigidity of the engine, or the increase in uneven temperature distribution.
Specifically, with the low rigidity of the engine or the increase of uneven temperature distribution, when the gasket is tightened, the unevenness of sealing surface pressure around the cylinder bore becomes large. Thus, the difference between portions where the surface pressure is high, and portions where the surface pressure is low, becomes large. Accordingly, gas leaks occur from portions where the surface pressure is low.
There are two main methods of solving a gas leak between cylinder bores. One method is improving the poor sealing condition of the corresponding part of the engine side. Another method is providing a gasket structure which can provide an adequate seal even under poor sealing conditions.
To solve the above-mentioned gas leak on the gasket side, conventionally, the total surface pressure of the sealing portion around the cylinder bore (combustion chamber) was increased, or, material with an elastic capability was used for a bead in order to reinforce the partially decreased surface pressure due to flexure and so on. Also, a reinforcing plate such as a shim or a comparable structure has been widely used.
At the same time, technology in which the sealing performance around the cylinder bore or the other sealing bores of the metal gasket is improved by a waveform bead (meandering bead) whose shape when viewed from a plan view is waved, is well-known. See, for example, Japanese Patent No. 3057445.
It is possible that the aforementioned meandering bead technology is applied between the cylinder bores, and that the meandering portion, where the main sealing bead meanders (i.e., meanders in a horizontal direction in a plan view), is disposed between the cylinder bores.
When a metal gasket with the above-described structure is used in a situation where the space between the cylinder bores is narrow, beads sealing adjacent cylinder bores are located close to each other. Accordingly, when the gasket is tightened, lower portions of the abutting beads contact each other and squeeze together. As a result, the lower portions of the beads become resistant to slippage resulting from repeated movement in the vertical direction of an engine member, such as the cylinder head and so on, which occurs during engine operation, even if there is bashing. Therefore, an appropriate sealing surface pressure can be maintained.
However, in the case of an engine which has a wider space between adjacent cylinder bores, the distance between meandering portions of the main bead widens, and a large gap is produced between both meandering portions. Due to the gap, maintaining the surface pressure becomes difficult. At the same time, when repeated motions in the vertical direction of the cylinder head occur during engine operation, the lower portions of the beads of the meandering portions gradually slip in a gap direction. Accordingly, the amount of creep (creep relaxation) of the beads becomes large. As a result, the sealing performance deteriorates.
The present invention was developed to solve the aforementioned problems associated with conventional metal gaskets. An object of the invention, therefore, is to provide a metal gasket capable to provide partially surface-pressure reinforcing measures in the outer portion of the main bead between the adjacent cylinder bores, and thus achieve excellent sealing performance relative to the cylinder bore.
Further objects and advantages of the invention will be apparent from the following description of the invention.